Catalysts are used in various fields, in promoting reactions, such as reactions for the synthesis of compounds and reactions of fuel cells, and also in cleaning automobile emissions. In many catalysts which have hitherto been used, porous materials, such as oxides of alumina, silica and the like and carbon, are used as a carrier, and precious metals, such as platinum, palladium and rhodium, are supported by the carrier. In recent years, multicomponent catalysts in which multiple precious metals are supported to improve activity have been growing popular. Also with respect to carriers, in catalysts for treating automobile emissions, recent years have seen the use of carriers to which there are added ceria having oxygen adsorbing and releasing abilities and a ceria-zirconia solid solution in which the oxygen absorbing and releasing abilities and heat resistance of ceria are improved in order to mitigate atmospheric variations of emissions.
These catalysts are usually manufactured by impregnating a porous oxide carrier with a metal salt solution, such as dinitrodiamine platinum, platinum chloride acid and rhodium nitrate, and performing calcination in a reducing atmosphere. Also, multicomponent catalysts are manufactured by preparing multiple metal salt solutions to be supported, mixing a carrier with the multiple metal salt solutions and causing multiple kinds of metal ions to be adsorbed on the carrier, and performing drying and calcination thereafter. In catalysts manufactured by these methods, during the impregnation with a metal salt solution, metal single particles in an atomic state are adsorbed on the carrier, and the single particles in an atomic state migrate and aggregate into catalyst particles due to heat treatment which is performed after that.
Patent Document 1: Japanese Patent Application Laid-Open No. 60-50491
Patent Document 2: Japanese Patent Application Laid-Open No. 63-116741
In recent years, the environment in which catalysts are used have been increasingly severe. For example, in catalysts for cleaning emissions, owing to the enhancement of emission control against the background of the global environment protection, examinations are being made as to installing a catalyst immediately under a manifold closer to an engine than before. In this case, the emission temperature becomes as high as not lower than 800° C. during high-speed travels of vehicles.
Therefore, although a catalyst capable of maintaining high activity for a long period even in a high-temperature atmosphere is sought after, there have hitherto been no catalysts that sufficiently meet this requirement. That is, in conventional catalysts, catalyst particles are released from the constraint done by a carrier in a high-temperature atmosphere and can migrate, and the catalyst particles which have migrated come into contact with other catalyst particles and aggregate with them, and become coarsened, with the result that the activity of the whole catalyst decreases. In particular, alumina, silica and the like which have hitherto been used as porous carriers have a low constraining force working on catalytic metals, and tend to allow catalytic metals to migrate due to the heat during the use of a catalyst, with the result that it is difficult to maintain activity.
In the field of catalysts, an improvement of activity is constantly required. Although in this respect there are many conventional catalysts having high activity, requests for the development of catalysts of high activity which exceed the conventional catalysts are high.
The present invention has been made against the above-described background and has an object to provide a catalyst which has high activity and can maintain its activity for a long period even in a high-temperature environment.